This invention relates to a new and improved mold assembly and a method by which it is made and more specifically to a segmented ceramic mold assembly which may advantageously be utilized in the casting of many different items. Among these items are turbine engine components such as diffuser cases, nozzle rings, vane assemblies, bearing supports and fan frames.
Relatively large turbine engine components, such as fan frames for turbojet engines, have previously been fabricated from a multitude of small castings, sheet metal panels and sections of machined bar forging. These various components are assembled into a jet engine fan frame having an annular hub or inner wall and an outer ring or wall which are interconnected by a plurality of struts or vanes. The struts are hollow to provide for deicing and to enable fluid conduits and other parts extending between the hub and outer ring to be enclosed within the struts. Certain known jet engine fan frames have a relatively large diameter outer ring, for example one particular jet engine fan frame has an outer ring of a diameter of more than forty inches. Heretofore, the casting of a one-piece jet engine fan frame having a relatively large diameter and the requisite dimensional tolerances has been extremely difficult if not impossible.
Relatively small diameter jet engine fan frames have been previously cast from one-piece ceramic molds which are formed by a lost wax process. This process involves the repetitive dipping of a wax pattern in a slurry of ceramic mold material and drying the material between the dip coatings. After a covering of a desired thickness has been built up on the wax pattern, the pattern is destroyed by melting and the mold is fired to have the desired strength. After firing, molten metal is poured into the mold to accurately form a cast part. The manner in which wax patterns are repetitively dipped and dried during the formation of a ceramic mold is well known and is disclosed in numerous patents, including U.S. Pat. Nos. 3,675,708; 3,422,880; 2,961,751; and 2,932,864.
Due to the fact that the wax patterns must be repetitively dipped in a body of liquid ceramic mold material, only relatively small patterns have been commonly utilized to form relatively small molds. Since the molds formed in this manner are integrally formed as one piece, it is extremely difficult, if not impossible, to detect and repair imperfectly formed interior mold surfaces. This can result in a substantial percentage of scrap even though only small parts are being formed. In addition, the closed integral nature of these known investment casting molds makes it extremely difficult to coat selected areas of the interior mold surface with inoculants which promote solidification of the metal poured into the mold in a desired manner.
To some extent, the difficulties resulting from the forming of a one-piece mold have been overcome by forming ceramic molds in a plurality of parts in the manner disclosed in U.S. Pat. Nos. 3,888,301; 3,802,482; 3,669,177; and 3,048,905. Ceramic type mold cores have been made in the manner disclosed in U.S. Pat. No. 3,675,708. In addition, molds of non-ceramic materials have been previously formed in a plurality of sections in the manner disclosed in U.S. Pat. Nos. 2,848,774 and 2,789,331. The cost of forming ceramic molds in the manner disclosed in at least some of the aforementioned patents is contributed to by the fact that an area between segments of a mold must be abraded or cut away to form the separate mold sections. Since the ceramic material forming the mold sections is extremely hard, this cutting away or abrading of the ceramic mold material is both difficult and time consuming.